Process for the treatment of powdered products



` May 13,v 1969 J. J. ciaorr 3,443,959

PROCESS FOR THE TREATMENT OF POWDERED PRODUCTS Filed March 11. 196s sheet of 4 IIJ May 13, 1969 J. J. claorr 3,443,959

PROCESS FOR THE TREATMENT OF POWDERED PRODUCTS Filed Marh 11. 1965 sheet 2 of 4 May 13, 1.969 J. J. clsolT 3,443,959

PROCESS FOR THE TREATMENT OF POWDERED PRODUCTS Filed Maron 11. 1965 sheet 3 or 4 /YTTOPA/f/J PROCESS FOR THE TREATMENT OF POWDERED PRODUCTS 30021050240 289 iw/901600930150 [l0 130 120 Il@ [00 95 JIM 7 4' TTOPA/[KS United States Patent O 3,443,959 PROCESS FOR THE TREATMENT OF POWDERED PRODUCTS Jacques Jean Ciboit, Paris, France, assignor to Etablissements Laguilharre, Courbevoie, Hauts-de-Seine, France, a corporation of France Filed Mar. 11, 1965, Ser. No. 438,991 Claims priority, application France, Mar. 13, 1964, 967,330; Mar. 1, 1965, 7,459 Int. Cl. A23c 9/08 U.S. Cl. 99-56 12 Claims ABSTRACT OF THE DISCLOSURE The wettability of dry powders is improved by producing a stream of the powder and at least one stream of a humidifying fluid which are substantially in parallel and coaxial relation, and a stream of drying air which is substantially parallel and coaxial with the first two streams and surrounding the first two streams, and rapidly contacting these different streams thus to instantaneously humidify the powder and immediately subsequently drylng.

The present invention has for its object a process for the treatment of powdered products which are derived especially from a spray-drying process and which are intended to be redispersed either in solution or in suspension for subsequent utilization as is the case, in particular, with milk-product powders. The invention also includes within its scope the devices for the practical applicatio-n of said process and, by way of novel industrial products, the powders which are obtained by means of said process.

It is known that certain powders, and especially sprayprocess powders, have poor wettability which makes it possible to disperse them in solution or in suspension only in an imperfect manner. A large number of processes have already been proposed for the purpose of increasing the wettability of such powders and thus to produce as a final result the so-called instantaneous powders, by subjecting them to an agglomerating treatment in the presence of moisture. Especially insofar as milk-product powders obtained by spray-drying process are concerned, the known processes can be classed in two main groups which are based on somewhat different principles, viz:

The processes of the first group make use of a finelydivided storage powder which is rehumidified in order to obtain agglomerates of substantial size which are subjected to a subsequent drying process. These processes have a disadvantage in that they are in practice only applicable to powders which have a high initial density and result in the production of instantaneous powders which have a low density (substantially of the order of one-half the initial density) and a heterogeneous particle-size distribution as well as high fragility. The apparatus employed for the treatment must in addition be kept under constant supervision by reason of the dangers of clogging which arise from excessive moisture content.

The processes of the second group are intended to obtain a powder which has a high moisture content directly at the delivery end of the spray-drying chamber and the grains of which are agglomerated and dried without any further introduction of moisture. The processes of this group have a disadvantage in that they also call for continuous supervision of the spray-drying apparatus by reason of the sticky nature of the powder which is prod-uced in this latter and which has a high moisture content. Although such processes do result in the production of powders of relatively high density, the wettability of said powders is nevertheless substantially lower than that of powders obtained by the processes of the first group.

While extending to the treatment of powders in general 3,443,959 Patented May 13, 1969 with a view to improving their wettability, the present invention applies more especially to the production of milk-product powders and in particular skimmed-milk powder; and the numerical values which are indicated in the following description relate to the manufacture of this product.

The treatment process in accordance with the invention makes it possible to obtain powders which are endowed with excellent wettability as well as a uniform particle-size distribution and the small-size agglomerates of which are not fragile and can readily be stored in bags, while the density of said powders is generally higher than that of powders which are obtained in conventional rehumidfying apparatus. Moreover, this process effects in a very short time perfectly uniform humidilflcation of the treated powder, thus subsequently calling only Ifor a relatively small addition of moisture which is compatible with stable industrial operation, which in turn eliminates the functional disadvantages of the known processes hereinabove referred-to.

To this end, the process in accordance with the invention essentially consists in producing a stream of powder to be processed and at least one stream of a humidifying fluid which are substantially in parallel and coaxial relation, and a stream of drying air which is substantially parallel and coaxial with the first two streams and surrounding said first two streams, and in rapidly contacting these different streams with a View to effecting instantaneously the humidification of the powder and immediate subsequent drying thereof, then in separating the processed powder from the air.

In certain cases, a stream of conditioned air which is substantially parallel and coaxial with the streams referred-to above can additionally be generated between the streams of powder and humidifying fluid and the stream of drying air and can also be rapidly contacted with all of the other streams.

The stream of powder can be produced either by means of a mechanical feed process of known type or preferably by putting the powder in suspension in a carrying airstream which is accordingly charged with a high proportion of powder.

A humidifying fluid is understood to mean any fluid which is charged with moisture and which is capable of imparting its moisture to the powder by being contacted with this latter. Said fluid can either be a gas or else can consist of a finely atomized aqueous liquid. In the case of a gaseous humidifying fluid, this latter can be, for example, steam, hot moisture-laden air, or a mixture of air and steam. In the case in which the humidifying fluid is an atomized liquid, said fluid can either be water or a solution or aqueous suspension of the same product as the powder to be processed, or alternatively a solution or aqueous suspension of another product which it may be found `desirable to mix with the powder to be processed.

If the humidifying fluid is a gas, the `stream of powder to be processed is preferably located inside the `stream of humidifying fluid and this latter surrounds the stream of powder in coaxial relation therewith, `although this arrangement is not an essential feature. lf the humidifying fluid is an atomized or vaporized liquid, this fluid will be located inside the -stream 4of powder and this latter will accordingly surround the humidifying lluid in coaxial relation therewith. Provision can also be made for two streams of gaseous lhumidifying fluid, one stream being located in the axis of the stream of powder and the other stream being located around this latter.

The stream of drying air which flows externally of and coaxially with the streams of powder and of humidifying fluid will preferably consist of hot air. If a lstream of conditioned air is additionally provided, said conditioned air can 'be at a temperature and possibly also have a degree of humidity which are variable according to the powders to be processed and the said stream of conditioned air can also be circulated at a velocity which is different from that of the humidifying fluid and/ or the drying air. In order that the humidilication of the powder should be uniform and be accompanied by a certain readhesion of the powder grains by collision, it is necessary to impart high iiow velocities to the humidifying iiuid and/or the powder stream. If the humidifying uid lis gaseous, the flow velocity thereof can be of the order of 50 to 200 meters per second, whilst lthe powdercarrying airstream which is charged in a proportion of kilograms of powder 4per cubic meter -of air, for eX- ample, will have a flow velocity of the order of 30 to 60 meters per second, for example. If the hum-idifying fluid -is an -atomized Iliquid, the powder-carrying airstream will have a high flow velocity, for example 50 to 250 meters per second.

In `the case in which the humidifying iiuid is a -gas and in which the powder is conveyed by a carrying airstream and the stream of humidifying fluid surrounds the powder stream in coaxial relation therewith, the powder-carrying airstream will preferably be cold and a rotational Iflow motion will preferably be imparted to said powder-carrying airstream in order to obtain a uniform distribution of the powder and to ensure that this latter is immediately and intimately contacted with the humidifying -uid as soon as the powder is in the presence of said iluid. It is intended in this manner to give greater effectiveness to the process -of uniform humidication of the powder grains and of adhesion of said grains to each other.

Since the humidiiication of the processed powder is carried out in a very short time followed Iby immediate drying, there is only a low momentary increase in the degree `of humidity :of the powder.

The processed powder can be a normal storage powder in a finely divided state and having an initial moisture content which is lower than 4% or alternatively a powder which is supplied directly and continuously from a spray-drying plant. The powder to be processed can in that case advantageously be a powder which is collected at the delivery end lof a spray-drier with a moisture content within the range -of 4.5 to 7% and preferably with-in the range of 4.5 to 6% in accordance with the process described in my U.S. patent application Ser. No. 434,617 tiled on Feb. 23, i965, now Patent No. 3,410,701, `and entitled Process for the Production of Milk-Product Powders, and Powders Made by This Process. This last-mentioned process consists in subjecting to a spray-drying process a concentrate containing a proportion of dry material which is higher than 45% and preferably -between 50 and 55% and having a temperature which is higher than 45 C. and preferably in the vicinity of 70 C., lby effecting the drying in the spraydrier until a powder is obtained having a moisture content as hereinabove indicated.

The process in accordance with the invention can also advantageously 'be applied to a powder which has been subjected to the double-drying operation described in the aforesaid U.S. patent application, that is to say a powder which has lbeen collected at the out-let of the spraydrier as has just been indicated and having a moisture (water) content within the range of 4.5 to 7% and preferably between 4.5 and 6%, and which has Athen been subjected to a secondary drying operation so as to reduce its moisture content to approximately 3.5%.

In all cases, after having been separated from the treatment air, the powder which is processed -in accordance with lthe invention can be subjected to a complement-ary drying and/or cooling process in `such a manner as to obtain an end product having a moisture content which is lower than 4% and a suiciently low temperature to permit of storage.

The processed powder can also be subjected to a screening or grading operation for the purpose of separating therefrom the fraction which has a particle size above a predetermined value and for the purpose of removing the finest particles therefrom so as to reintroduce them if necessary at a -suitable stage of `the processing cycle. The screening or grading operation can if necessary be combined with the drying and/or cooling operation. The fraction thus obtained which has the llargest particle size constitutes a high grade instantaneous powder.

In the simplest form thereof, the device which is provided 'by the invention for the practical application of the process described above essentially consists of Ia kind of combining-tube comprising two coaxial nozzles -through which are canalized respectively the stream `of powder and the stream of humidifying air and a third nozzle which is located outside said lirst two nozzles in coaxial relation thereto and through which the drying air Iis conveyed, said ythird nozzle Vbeing extended by a pipe which is connected to a separator of the cyclone type, for example, `in which the processed powder is separated from the air.

When provision is made for a conditioned air stream, the device comprises a Vfourth nozzle which is placed between the first two nozzles and the third nozzle for the purpose of canalizing said conditioned airstream.

The different nozzles which constitute the device are preferably given a tapering frustoconical shape at the outlet extremities thereof so as to have at this point a minimum cross sectional area and thus to increase the discharge velocities of the streams which are fed into said nozzles respectively. The pipe for connecting the outer nozzle which is supplied with drying air to the separator which serves to separate the powder from the air can be cylindrical or slightly divergent.

In the case in which the powder is conveyed Aby an air stream, the inlet of the nozzle which serves to canalize said stream can be coupled to a cylindro-conical chamber provided with a tangential inlet through which the powder is introduced, thereby imparting thereto the vortical ilow motion referred-to above. In all cases, the humidifcation and drying device which 1s constructed in one of the forms indicated above can be coupled or combined with a spray-drying plant for the purpose of processing the powder as this latter is delivered from said plant.

Itv has been observed that the advantageous properties of powders obtained by means of the treatment process and devices according to the invention are particularly marked when said process and said devices are applied to the treatment of powders prepared in accordance with the aforesaid U.S. patent application, namely powders as collected at the outlet of the spray-drier, that is to say having a moisture content which is preferably within the range of 4.5 to 6%, or after said powders have been subjected to a secondary drying process so as to reduce their moisture content to approximately 3.5%.

These properties of powders which are obtained in accordance with the invention have been analyzed by the present applicant, especially in regard to commercially available skimmed-milk powders of the type known as instantaneous powders. This analysis was concerned especially with the following characteristics:

(l) Volume of interstitial spaces existing in a given weight of powder as a function of the bulk density of the powder, the value of this volume being referred-to as porosity in the remainder of this description as well as in the claims appended hereto.

(2) Index of solubility according to the standards laid down -by the organization known as the American Dry Milk Institute (ADMI).

(3) Wettability or time of dispersion of a predetermined quanity of powder which has been poured over the free surface of a certain volume of water at a predetermined temperature.

(4) Self-dispersion or spontaneous dissolving test without agitation of a predetermined quantity of powder in water at a given temperature with measurement of the quantity of dry material which has passed into solution in a given time.

(5) Inuence of the porosity as dened under Section 1 hereinabove on the fragility of the aggregates, on the wettability and on the self-dispersion of the powder.

The results of these tests as well as other characteristic features of the invention will `be brought out by the complementary description which follows below, reference being made to the accompanying drawings which are given solely by way of nonlimitative example, and in which:

FIG. 1 is a diagrammatic cross section of a simple form of construction of a humidication and drying combiningtube in accordance with the invention;

FIG. 2 is a diagram of the complete installation for the execution of the treatment process in accordance with the invention;

FIG. 3 is a diagrammatic cross section of a humidication and drying combining-tube which is provided with a fourth nozzle for the admission of conditioned air;

FIGS. 4 and 5 are diagrammatic cross sections of two alternative forms of construction of the combining tube of FIG. 1;

FIG. 6 is a diagrammatic cross section of the test-tube which is employed for measuring the porosity of a powder as a function of the bulk density of said powder;

FIG. 7 is a diagram representing the results of actual measurements taken on a number of different powders of instantaneous skimmed milk;

FIG. 8 is a diagrammatic cross section of the equipment employed for the purpose of carrying out wettability and self-dispersion tests;

FIG. 9 is a diagram representing the results of comparative tests performed on .powders of instantaneous skimmed milk available on the market and on skimmed milk powders according to the invention, in regard to the variations in wettability and in self-dispersion of the powders as a function of their porosity.

In the simple form which is illustrated in FIG. 1, the humidiiication and drying combining-tube which is designed for the execution of the process in accordance with the invention consists of two coaxial nozzles 1 and 2 for the canalization of streams of powder and humidifying fluid and a single peripheral nozzle 3 for the canalization of the drying air.

The powder is preferably conveyed by a stream of carrying air and fed through the central nozzle 1 whilst the humidifying fluid which is gaseous in this case and which consists, for example, of steam, is introduced at 2a into the nozzle 2. Said nozzle 2 advantageously has a frustoconical portion which terminates in a discharge end of smaller diameter and the humidifying fluid is introduced therein at a certain pressure so as to be endowed with a high discharge velocity which is preferably comprised between 50 meters per second and 200 meters per second. The central nozzle 1 which conveys the powder has in this case a cylindrical shape but can also comprise a frustoconical portion which forms a discharge outlet of smaller diameter; the discharge velocity of the powder stream can vary between 30 and 60 meters per second.

The drying air which is admitted at 3a is in principle hot air, for example at a temperature of approximately 140 C. The nozzle 3 which canalizes said hot air has in this case a convergent rustoconical shape and is extended -by a conduit 4 which terminates in a separator for separating the powder from the air (not shown in FIG. 1). The conduit 4 consists in this example of a portion of divergent frustoconical shape followed by a cylindrical portion 4a but could also be cylindrical over its full length. It is in said conduit 4 that the powder and humidifying fluid which pass out of the nozzles 1 and 2 are contacted with each other and with the drying air canalized by the nozzle 3 and are intimately mixed therein, and in said conduit also that the instantaneous humidication of the powder and subsequent drying thereof consequently take place. The rate of mixing within said conduit is preferably within the range of 10 to 2O meters per second. The length of the conduit can be as desired but is preferably between 1 and 3 meters.

In the diagram of the installation which is illustrated in FIG. 2, there is again shown at A the humidification and drying combining-tube of the type which is illustrated in FIG. 1. The nozzle 1 which serves to inject the stream of powder is connected at the upstream end thereof to a cylindro-conical chamber 5 to which the powder supply duct 6 is connected tangentially in order to impart a vortical flow motion to the powder-charged airstream. The duct 6 starts, for example, from a device 7 in which the powder to be processed is discharged from a hopper 8 and is put in suspension in a stream of compressed air which is fed in at 9. The powder can be, for example, a storage skimmed-milk powder having a moisture (water) content which is less than 4%. The supply of powder to the hopper 8 and the delivery of compressed air which is admitted at 9 can be regulated, for example, in such a manner that the proportion of powder with which the air stream is charged is of the order of 10 kilograms per cubic meter of air.

The humidifying fluid which is introduced at 2a into the nozzle 2 of the combining-tube consists, for example, of steam derived from a suitable source or of a mixture of steam and air, for example a mixture of steam and air in equal proportions by weight. This fluid is admitted within the nozzle 2 under a pressure such that its velocity at the outlet of said nozzle is preferably comprised within the above-mentioned limits of 50 to 200 meters per second, whilst the velocity of the powder at the outlet of the nozzle 1 is preferably comprised within the limits also given above of 30 to 60 meters per second.

The drying air is admitted at 10 in the form of cold air and is directed by a fan 11 into a heating battery 12 in which its temperature is brought, for example, to C. and is admitted at 3a into the peripheral nozzle 3 of the combining-tube A. The flow rate of the drying air can be, for example, between 1500 and 2000 cubic meters per hour for the purpose of processing 500 kilograms per hour approximately of powder.

There is shown at 13 a separator of the cyclone type in which the gaseous mixture together with the powder delivered by the combining-tube A is conveyed through the conduit 4 which, in this example, is cylindrical throughout its length. The air which is separated from the powder is discharged from the top of the separator 13 by means of a fan 14 and the processed powder is withdrawn from the bottom of the separator by means of a drum 15 and discharged, for example, onto a vibratory refrigerating table 16 on which the powder is cooled by a flow of cold air which is driven by a fan 17 and subjected to a classifying or grading process. The fraction having the largest particle size is collected at 18 and accordingly constitutes an excellent instantaneous powder which has a relatively ner particle size and higher density than the instantaneous powders which are at present known. On the other hand, the fines are sucked out and directed through a pipe 19 into a cyclone separator 20 which is coupled to a fan 21. The air which is separated from the nes is discharged at the top of the separator 20 and the iines are extracted at the bottom of this latter and discharged through a drum 22 into the hopper 8 so as to be reincorporated in the powder to be processed.

The table 16 could comprise a rst section which is supplied with hot air so as to produce a complementary drying of the powder as contemplated above.

FIG. 3 illustrates a processing combining-tube which is similar to the tube of FIG. 1 but which, apart from the nozzles 1, 2 and 3 which serve to canalize the streams of powder, humidifying Huid and hot drying air, is also provided with a fourth nozzle 23 for the admission of conditioned air which is fed in at 23a and which, depending on the powders to be processed, can ybe either hot or cold. In this example, the four nozzles which are disposed in coaxial relation, are endowed at the discharge end thereof with a convergent frustoconical shape for the purpose previously stated. In addition, as in the example of application which is illustrated in FIG. 2, the nozzle 1 through which the powder in suspension in an airstream is canalized is connected at its upstream end to a cylindro-conical chamber fitted with a tangential intake 6 for the purpose of imparting a vortical flow motion to said airstream.

In the alternative form of embodiment` which is illustrated in FIG. 4 which is intended for the utilization of a humidifying fluid consisting of a liquid, this latter is canalized through the central nozzle 1. In this case, the discharge end of the nozzle 1 could be fitted with a spraying nozzle to which the liquid could be brought under pressure. As a preferable feature, the mixture of air and powder which is conveyed through the nozzle 2 will be brought into this latter under a certain pressure in such a manner as to obtain at the outlet of said nozzle a high rate of expansion permitting the atomization of the liquid as this latter is discharged from the nozzle 1. The velocity of the powder-carrying gas-flow will thus be high and preferably between 100 and 250 meters per second, which will accordingly assist collisions between the powder grains and droplets of liquid.

As an alternative form, the spray atomization of the liquid could be produced by means of an auxiliary jet of compressed air which is brought to the outlet of the nozzle 1 in a manner which is known per se.

FIG. 5 represents another alternative form of embodiment of the humidiication and drying combining-tube in which the nozzle 1 which conveys the powder-carrying gas-flow is disposed coaxially between two concentric nozzles 2 and 2 for receiving the humidifying fluid which, in this case, consists of a gaseous fluid. The drying air nozzle 3 is disposed, as in the previous examples, at the periphery of the assembly. The nozzle 1' to which the powder is supplied is connected, as previously stated, to a cylindro-conical chamber 5 which is provided with a tangential intake 6 for the admission of powder.

As has also been explained earlier, the humidification and drying device as designed in one of the forms which have been described or in an equivalent form can be coupled with a spray-drying installation for the purpose of processing the powder as this latter is produced in said installation. This application proves to be particularly effective for the direct and continuous processing of a powder which is collected at the delivery end of a spray-drier and has a moisture content in the range of 4.5 to `6% in accordance with the process described in the aforesaid U.S. patent application, but can also be utilized for the direct and continuous processing of a normal finely-divided powder which is collected at the delivery end of the spray-drier and having a moisture content below 4%. In both cases, the hot drying air and/or conditioned air which is fed to the humidiication and drying combining-tube can be taken directly from the spray-drying plant. The air which carries the powder to be processed can be drying air or conveying air which is derived at the same ltime as the powder from the separator of the spray-drying plant or from one of the separators if provision is made for a number of these latter. The delivery of the fan located at the outlet of the separator of the humidiication plant (for example the separator 13 of FIG. 2) can be returned to any suitable point of the Spray-drying plant upstream of the spray-drying plant separators (either into the spray chamber or 1nto the pipes upstream of the separators) so as to effect a recovery of the fines.

However, the application of the process and devices in accordance kwith the invention iS. not limited to a direct and continuous treatment of a finely divided powder which is collected at the delivery end of a spray-drier but, as has also been indicated above, said process and device can also be applied to the treatment of finely-divided storage powders which have a moisture content below 4% and more especially of powders which have undergone the double-drying process contemplated in the aforesaid U.S. patent application.

The results of tests performed by the present applicant with a View to determining a certain number of advantageous properties of powders obtained according to the invention will now be set forth hereinafter.

I. Measurement of bulk density and porosity For the purpose of determining the bulk density of a given powder, the present applicant has employed a cylindrical glass test-tube of cc., graduated in cc. and having an internal diameter of approximately 25 millimeters. The method adopted was as follows:

Approximately 50 cubic centimeters of powder to be analyzed were introduced in the test-tube which was previously calibrated, by causing the powder to slide into the test-tube (without pouring) by means of a smooth sheet of paper which was perfectly dry. During this operation, the powder which is placed on the sheet of paper must not be shaken or compressed.

The test-tube was dropped three times from a height of 2.5 centimeters at two-second intervals while avoiding any jerks when lifting the test-tube. The test-tube was dropped onto a hard wood surface. This falling motion must be uniform and accurately timed.

The test-tube was then allowed to rest for a period of thirty seconds, whereupon the volume taken up by the ptowder within the test-tube was then read, the testtube containing the sample was weighed and the exact weight of the test sample was then determined.

The bulk density of the powder is expressed by the ratio:

weight in grams of the test sample volume of the test sample in cubic centimeters It was found preferable to adopt the mean value of the results obtained in three measurements effected on samples of the same powder.

The porosity of a milk powder, defined earlier as the volume of open spaces between the powder grains, could be calculated on the basis of the bulk density of the powder, the density of the solids constituting the powder and the volume of occluded air within the grains. The present applicant has adopted a similar method which consists in measuring the volumes of the open spaces between the grains of powder which is immersed in petroleum ether. This method has been adopted on account of the good reproducibility of measurements.

The equipment employed consists of a cylindrical glass test-tube having an internal diameter of 25 millimeters and graduated in cc. up to cc.

The teso-tube is shown diagrammatically at 24 in FIG. 6. There were poured into said test-tube 85 cm.3 of petroleum ether of the class 40-65" at a temperature of 20 C. The level reached by the petroleum ether within the test-tube is shown in the ligure at a.

A quantity of 25 grams of powder to be analyzed was placed on a sheet of smooth paper which was perfectly dry and the powder was slowly introduced into the testtube 24 by causing said powder to slide (without pouring) down the sheet into the test-tube. The time of introduction must be of the order of two minutes, the introduction being continuous and uniform and all precautions being necessarily taken to avoid any shaking or compression of the powder while this operation is in progress. In the event of formation of an arch at the surface of the petroleum ether, the outer surface of the test-tube can in that case be tapped lightly.

After a period of five minutes during which the test- Si tube was allowed to rest, the level b attained by the powder in the bottom of the test-tube and the level c attained by the petroleum ether were noted. As can thus be readily understood, the volume which is comprised between the level c and the initial level a of the petroleum ether corresponds to the volume of the powder grains whilst the lower volume which is defined by the level b corresponds to the sum of the volume of the grains and the volume of free spaces between the grains within the petroleum ether. Since the volume of petroleum ether which is introduced in the test-tube is 85 cubic centimetres and if the reference P25 designates the open porosity, that is to say the volume of free spaces which are formed between the grains of the 25 grams of powder which constitute the sample under analysis, these facts can be represented by the relations:

Volume of the grains=c85 and Volume of the grainszb-P25 hence c- 85 b -P25 and In relation to 100 grams of powder, the porosity P100 is therefore expressed by the relation:

It is therefore essential to ensure that the petroleum ether completely covers the powder introduced in the test-tube. Were this condition not complied with, it would be necessary to carry out the operations described above with a lower weight of powder such as 20 grams, for example. In this case, the relation which gives the porosity would be:

A series of measurements taken by this method, on the one hand on skimmed milk powders available in commerce and on the different products obtained from these powders by successive bolting operations in which use is made of screens having progressively decreasing mesh sizes and, on the other hand, on skimmed milk powders obtained in accordance with the invention, have served to establish that the porosity P of these products as a function of their bulk density is substantially comprised between the two curves A and B of the diagram shown in IFIG. 7, the values of the bulk density being plotted as abscissae and the values of porosity in cubic centimeters per 100 grams of powder being plotted as ordinates.

The influence exerted by the variations in open porosity as thus determined on the characteristics of wettability and of self-dispersion of the powders will be set forth hereinafter.

II. Index of solubility according to ADMI standards According to the standards laid down by the American Dry Milk Institute (ADMI), the index of solubility of an instantaneous milk powder, as determined in accordance with the method defined by said standards by dissolving in distilled water at 24 C. in the case of skimmed milk powders and at 60 C. in the case of powders containing fats such as whole milk powders, must be lower than or at least equal to one milliliter. It has in fact been found that, generally speaking, the treatment process in accordance with the invention does not modify the index of solubility of the powder prior to processing. In order to satisfy the condition laid down by the ADMI standards, it is therefore advisable and preferable for the application of the process according to this invention to make use of a starting powder which has in turn an index of solubility which is lower than or at the most equal to one milliliter and preferably lower than 0.3 milliliter.

III. Wettability and self-dispersion It is `known that the quality of instantaneization f a milk powder is characterized especially by the values of wettability and self-dispersion of said powder, these properties having been dened earlier.

Tests carried out by the present applicant with a View to measuring these values in the case of different skimmed milk powders have been performed in distilled water at 40 C., this temperature having been chosen by reason of the etsablished fact that skimmed milk powder has maximum solubility at this temperature. When these tests are performed on powders containing fats such as whole milk powders, the water employed must be at a temperature of 60 C.

The equipment employed for these tests consisted (as shown in FIG. 8) of a stationary screen 25 having a mesh size of 6 millimeters and placed above a Buchner funnel 26 having a diameter of 85 millimeters and fitted at the bottom end with a rubber tube 27 closed by means of a clip 28 and containing 100 cc. of distilled water at 40 C., the screen 25 being placed at a distance of 80 millimeters above the level of the water. There was placed beneath the tube 27 a wire-mesh filter 29 having a mesh size of 200 microns and xed within an ordinary funnel 30, the stem of which was placed within a graduated glass test-tube 31 having a capacity of 120 cc.

The mode of operation was as follows:

The screen 25 was carefully dried by lightly heating so as to prevent any sticking of the powder to the surface of the meshes. Then, cubic centimeters of distilled water at 40 C. (in the case of skimmed milk powders) were poured into the Buchner funnel 26 and the temperature was balanced when necessary by heating the funnel 26 with a Bunsen burner, l0 grams of powder to be analyzed being then poured onto the screen 25. The time taken by the powder to disappear by immersion and/ or dissolving in the water contained in the Buchner funnel 26 constitutes the test for determining the wettability of said powder. Said time is measured from the moment when the powder is initially poured onto the screen 25 (this pouring operation being performed very rapidly) up to the moment when the powder has completely disappeared from the surface of the water.

After three minutes have elapsed, the `clip 28 was opened and the product was allowed to flow out into the filter 29 and into the test-tube 31 located underneath the funnel 30. The outflow time must not exceed one minute.

The filling of the test-tube 31 was then completed with distilled water up to its full capacity of cc.; the testtube was turned upside down so as to homogenize its contents and the weight of dry extract of the filtered liquid was then measured. This weight of dry extract makes it possible to determine the percentage of weight of powder which has passed into solution. It is this percentage which expresses the self-dispersion of the powder.

There are shown in the diagram of FIG. 9 the results of a series of wettability and self-dispersion tests which have been performed by the present applicant on the one hand on instantaneous skimmed milk powders available in commerce and on their bolting products and, on the other hand, on powders obtained by applying the process in accordance with the invention and the device shown in FIG. 2 to the treatment of skimmed milk powders produced in accordance with the process described in the aforesaid U.S. patent application, either prior to or after the secondary drying operation which is envisaged in said application. These tests were performed in respect of different values of porosity of the different powders analyzed as measured in accordance with the method hereinabove described. In the diagram of FIG. 9, the porosity values in cubic centimeters per 100 grams of powder have been plotted as abscissae whilst the self-dispersion Values have been plotted as ordinates on the left-hand side of the diagram, and the wettability values have been plotted as ordinates on the right-hand side of the diagram.

Curve C represents the maximum values of self-dispersion of commercially available instantaneous skimmed milk powders which have been subjected to the tests and curve D represents the minimum wetting times of the same powders. It is observed that the values indicated by these curves in respect of a porosity P ranging between 350 cubic centimeters and 100 cubic centimeters correspond to the values given by the following empirical formulae:

Self-dispersion as percentage=97(450/P)2'15 and wetting times in seconds=6+0.5(450/P) 275 A comparative analysis of the results which are shown in this diagram gives rise to the following observations:

In the first place, a fact worthy of note is that the instantantous skimmed milk powders which are available on the market have a very low bulk density. This bulk density, which is measured according to the method described above, is usually lower than 0.35. In practice, these powders have at the same time a porosity which is distinctly higher than 200 cubic centimeters in respect of 100 grams of powder. Their self-dispersion in distilled water at 40 C., which is usually lower than the maximum Values indicated by the top portion C1Cx of curve C of the diagram of FIG. 9 is comprised within the range of 85 to 92% and their wetting time, also in distilled water at 40 C., is higher than the values indicated by the bottom portion DlDx of curve D and ranges from 8 to 30 seconds.

It is also known that the aggregates which constitute the commercially available powders referred-to are very fragile. As a result, said powders must be sold in rigid cans or drums and cannot be delivered in bags inasmuch as any compression breaks up the aggregates and considerably reduces their properties of instantaneization. It is also found that, during handling operations and during storage, a self-sorting process tends to take place, with the result that the heaviest particles remain at the top of the cans and the nest particles collect at the bottom. lt can thus be visualized that the fractions which are thus separated have different instantaneization properties, The same applies if a particle-grading process is carried out, as indicated above for the purpose of measuring the porosity, by passing said powders through screens having predetermined mesh sizes in such a manner as to remove the largest particles. That fraction which consists of grains having the smallest sizes as obtained by screening no longer has the same properties of instantaneization as the initial powder, whereas the bulk density thereof increases and its porosity decreases.

These facts are clearly brought out by the curves C and D. It can be seen from these curves that, when the porosity of samples taken from a commercially available instantaneous skimmed milk powder ranges from approximately 300 cubic centimeters per 100 grams of powder to 100 cubic centimeters, their maximum self-dispersion drops correlatively from 93% to 35% and their minimum wetting time increases correlatively from eight seconds to approximately forty seconds.

In conclusion, the present applicant has; found that none of the instantaneous skimmed milk powders at present known and which have been subjected by him to the tests described exhibits both a wetting time which is shorter than the values indicated by curve D and a self-dispersion which is higher than the values indicated by curve C.

Similar tests performed by the present applicant on skimmed milk powders processed in accordance with the invention reveal that, in respect of equal porosity, these powders exhibit properties of wettability and self-dispersion which are considerably improved with respect to known powders. By way of example, in a series of tests performed on powders obtained directly by means of the process according to the invention, with porosities ranging from 300 to 80 cubic centimeters in respect of 100 grams of powder, the value of measured self-dispersion are represented by curve C; it is found that all of these values are distinctly higher than the maximum Values indicated by curve C in respect of commercial-grade skimmed milk powders which have corresponding porosi- 12 ties. The wetting times of the same powders as obtained in accordance with the invention and which also have porosities ranging from 300 to cubic centimeters are represented by curve D: this latter shows that these times are always distinctly shorter than the maximum wetting times indicated by curve D in the case of commercial-grade instantaneous skimmed milk powders having corresponding porosities.

The application of the treatment process in accordance with the invention to skimmed milk powders obtained in accordance with the process descirbed in the aforesaid U.S. patent application accordingly makes it possible to obtain a range of products having both shorter wetting times than those indicated by curve D and higher selfdispersion values than those which are indicated by curve C.

Moreover, it is apparent from curves C' and D that the powders in accordance with the invention also have high characteristics from the point of view of self-disper- Sion and wetting time in the case of porosity values below cubic centimeters per 100 grams of powder; these porosity values are not met with in instantaneous skimmed milk powders which are available on the market. In the case of these porosity values, the powders according to the invention have wetting times of less than two minutes and self-dispersion values which are higher than 35%.

Furthermore, the present applicant has observed that, in the case of a porosity below 200 cubic centimeters per 100 grams of powder, the products in accordance with the invention have a degree of fragility during manufac ture, handling, transportation and storage which is considerably lower than that of instantaneous s-kimmed milk powders which are sold on the market. The products manufactured in accordance with the invention can even be delivered in bags without any appreciable reduction in their properties. It would appear that the lower degree of fragility mentioned above is not due to a difference in individual fragility of the aggregates which constitute the powder but to the fact that the product which is processed according to the invention has a more uniform particle size distribtuion and, consequently, a lesser tendency to self-grading.

It is also apparent from a study of the diagram of FIG. 9 that, in the case of porosity values comprised between 200 and cubic centimeters, the products in accordance with the invention have a wettability and self-dispersion of the same order of magnitude as those of instantaneous skimmed milk powders which are at present offered on the market and have considerably higher porosity values.

The above-noted findings relate more especially to the skimmed milk powders which are obtained in accordance with the invention. Similar improvements have been made by the present applicant in the more general eld of milk product powders and especially milk powders containing fats when they have been processed by the method according to the invention.

What I claim is:

1. A process for the treat-ment of powdered material, particularly milk product powders, which comprises the steps of:

feeding a stream of a powdered material into one end of an elongated conduit in a direction extending substantially parallel to the lengthwise axis of said conduit;

simultaneously therewith directing a stream of a humidifying fluid into said one end of said conduit in a directon substantially parallel and coaxial with said stream of powdered material with one of said streams being disposed in surrounding relation to the other stream;

simultaneously therewith directing a stream of drying air into said one end of said conduit in a direction substantially parallel and coaxial with said powdered material product stream and said humidifying fluid stream with said drying air stream being disposed in surrounding relation to both of the other two streams; effecting ow of said streams through said conduit axially thereof and permitting said streams to mix within said conduit and thereby contacting said humidifying uid stream with said powdered material stream to increase the moisture content of the powdered material and simultaneously contacting the drying air stream with the other two streams to effect an immediate subsequent drying ofthe powdered material whereby the moisture content of the powdered material is momentarily increased in order to form agglomerates of the powdered material following which the powdered material agglomerates are immediately dried in order that the agglomerates formed have a relatively small size, a relatively uniform particles size distribution, an improved resis- ,ance to disintegration and an increased density;

removing the powdered material suspended in a uid from said conduit at the other longitudinal end thereof and then, outside of the conduit, separating the powdered material from the uid in order to recover the powdered material.

2. Process in accordance with claim 1, wherein a stream of conditioned air which is substantially parallel and coaxial with the aforesaid streams of powdered Imaterial, humidifying uid and drying air is generated between the streams of powdered material and humidifying iluid and the stream of drying air and is also contacted with all of the other aforesaid streams.

3. Process in accordance with claim 1, wherein the stream of powdered material is obtained by putting the powdered material in suspension in a carrying airstream.

4. Process in accordance with claim 1, wherein the powdered material to be processed is a normal storage powdered material in a nely divided state and having an initial moisture content which is lower than 4%.

5. Process in accordance with claim 1, wherein the powdered -material to be processed is obtained by subjecting to a spray-drying process a concentrate containing a proportion of dry material which is higher than 45% and preferably between 50 and 55% and having a temperature which is higher than 45 C. and preferably in the vicinity of 70 C., and which has been collected at the delivery end of the spray-drier, with a moisture content ranging from 4.5 to 7% and preferably from 4.5 to 6%. 6. Process in accordance with claim 1, wherein the humidifying iluid is a moisture-charged gaseous uid which is capable of imparting its moisture to the powdered material by being contacted with said powdered material.

7. Process in accordance with claim 6, wherein the stream of powdered material to be processed is located inside the stream of gaseous humidifying fluid which surrounds the stream of powdered material in coaxial relation therewith.

8. Process in accordance with claim 1, wherein provision is made for two streams of gaseous humidifying fluid, one stream being located in the axis of the stream of powdered material and the other stream being located around said stream of powdered material.

9. Process in accordance with claim 1, wherein the coaxial stream of drying air which flows outside the streams of powdered material and humidifying fluid consists of hot air.

10. Process in accordance with claim 1, wherein high ow velocities are imparted to the humidifying uid and to the stream of powdered material.

11. Process according to claim 1, wherein all of said streams ilow in a substantially horizontal direction.

12. Process in accordance with claim 1, wherein said stream of powdered material has a ow rate within the range of 30 to 60 meters per second and said stream of humidifying tluid has a flow rate within the range of 50 to 200 meters per second, and wherein the ow rate of the streams in said conduit is within the range of 10 to 20 meters per second.

References Cited UNITED STATES PATENTS 2,832,686 4/1958 Loudet et al 99-56 2,835,586 5/1958 Peebles 99-56 3,065,076 1l/l962 Wenner et al. 99-56 A. LOUIS MONACELL, Primary Examiner.

D. M. NAFF, Assistant Examiner.

U.S. Cl. X.R. 

